1. Field of the Invention
The invention relates to a force sensing device, and especially, but not solely, a device for sensing an oscillating force.
Micro-electro mechanical system (MEMS) oscillation sensing devices which convert an oscillating force (or energy) into electric signals are required for a growing number of portable or mobile communication devices. Such MEMS devices include microphones and frequency filters.
2. Description of the Related Art
The majority of MEMS microphones are based on capacitive detection principles. Microphones of this type typically consist of two parallel plates. One of the parallel plates is a compliant diaphragm and is moved when it is subjected to acoustic energy. Hence, the mechanical vibration of the compliant diaphragm caused by the acoustic energy results in a change of capacitance between the two plates. This change in capacitance is detected and output as an electrical signal.
Unfortunately, such capacitance devices generally rely on electrostatic detection which requires a high DC bias voltage. In addition, as the sensitivity of the sensor depends only on the change in the capacitance, it is difficult to design and manufacture high sensitivity devices.
Furthermore, this conventional type of device is fabricated by cutting silicon wavers for each individual device. Therefore, it is difficult or impossible to adjust the sensitivity of the device after fabrication.
In accordance with the present invention, a force sensing device comprises a magnetised member mounted on a substrate, a magneto-electrical material mounted on the substrate such that the magneto-electrical material is subjected to the magnetic field generated by the magnetised member, a movable member mounted for movement in response to an applied force, movement of the movable member causing a change in the magnetic field experienced by the magneto-electrical material, and an electrical property of the magneto-electrical material changing in response to the change in the magnetic field experienced by the magneto-electrical material due to the movement of the movable member.
In the context of this application, the term xe2x80x9cmagneto-electrical materialxe2x80x9d includes any material in which an electrical property of the material changes in response to changes in a magnetic field to which the material is exposed.
Preferably, the magneto-electrical material may comprise a magneto resistive material for example, an anisotropic magneto resistive material, a giant magneto resistive (GMR) material or a tunnel magneto resistive (TMR) material. Alternatively, the magneto-electrical material may comprise a Hall effect material, an inductive sensor or a magneto impedance sensor.
Preferably, the force sensing device may be a micro-electro mechanical system and may be formed on a single chip or die formed from a suitable substrate, such as a silicon wafer. In this case, preferably, the magnetised member comprises a permanent magnetic film formed on a portion of the substrate and the magneto-electrical material is also in the form of a thin film formed on the substrate, such as a magneto resistive alloy film. Typically, the permanent magnetic film may be a cobalt alloy or an iron alloy. Examples of suitable alloys include CoCr, CoPt, CoCrPt, CoCrTa and xcex3-Fe2O3.
In one example of the invention, the magnetised member is mounted on the movable member, so that movement of the movable member moves the magnet to generate the change in the magnetic field to which the magneto-electrical material is exposed.
In another example of the invention, the magnetised member and the movable member may be separate, and typically, the movable member may comprise a soft (or non-permanent) magnetic material.
Typically, the force sensing device is for sensing an oscillating force. However, it may also be used to sense non-oscillating forces, such as acceleration or changes in air pressure.
Preferably, the movable member may be in the form of a cantilevered member or a flexible diaphragm.
If the movable member is intended to undergo oscillation in response to an oscillating electrostatic force generated by an applied oscillating electrical signal (for example, if the force sensing device is to act as a frequency filter) the movable member may comprise a cantilevered member, a pivoting member or a linearly movable member. Typically, the magnetised member is mounted on the movable member.
Alternatively, the magnetised member may be fixed in position relative to the magneto-electrical material and the movable member, comprising a nonpermanent magnetic material, is mounted between the magnetised member and the sensor to move in response to the applied oscillating electrical signal relative to the magnetised member and the magneto-electrical material to generate the change in the magnetic field to which the magneto-electrical material is exposed.
Preferably, where the force sensing device is intended to be used as a frequency filter, the movable member has a resonant frequency, the resonant frequency corresponding to the frequency which the filter is intended to pass.
Typically, two or more force sensing devices may be combined to form a band pass filter where the movable member of each sensing device has a different resonant frequency.